1. Field of the Invention
The present invention relates to a method and circuit for synchronizing input and output synchronization signals and more particularly, to a method and circuit for synchronizing input and output synchronization signals, which can synchronize an output synchronization signal based on frequency change of an input synchronization signal and limit input and output periods, thereby preventing flickering, a backlight driver of a liquid crystal display device using the same, and a method for driving the backlight driver.
2. Discussion of the Related Art
Representative examples of flat panel display devices that display images using digital data include Liquid Crystal Display (LCD) devices using liquid crystals, Plasma Display Panels (PDPs) using discharge of inert gas, and Organic Light Emitting Diode (OLED) display devices using OLEDs. Among these, LCD devices have been widely applied to a variety of fields, such as TVs, monitors, laptop computers and cellular phones.
A liquid crystal display device is configured to display images via a matrix of pixels that use electrical and optical properties of liquid crystals having anisotropy, such as refraction and permittivity. Each pixel of the liquid crystal display device performs gradation by adjusting transmittance of light through a polarizing plate via change in the alignment direction of liquid crystals depending on data signals. Such a liquid crystal display device includes a liquid crystal panel to display images via a matrix of pixels, a drive circuit to drive the liquid crystal panel, a backlight unit to irradiate light to the liquid crystal panel, and a backlight driver to drive the backlight unit.
Recently, an LED backlight unit whose light source is light emitting diodes (hereinafter, referred to as LEDs) has been used because LEDs have advantages of more rapid lighting operation, higher brightness and lower power consumption than conventional lamps. The LED backlight unit emits white light generated using white LEDs or a combination of red/green/blue LEDs. Moreover, the LED backlight unit can advantageously perform not only global dimming that controls backlight brightness throughout the backlight unit, but also local dimming that controls backlight brightness on a per position basis, i.e. on a per split block basis.
A backlight driver to drive the LED backlight unit functions to generate a Pulse Width Modulation (PWM) signal having a duty ratio corresponding to a dimming value input from an external system, such as a TV set, or a timing controller, and adjust a turn-on/turn-off time of the LED backlight unit based on the PWM signal to control brightness of the LED backlight unit.
The backlight driver utilizes a Vertical Synchronization (VSYNC) signal that divides a frame of image data input from the external system to synchronize the LED backlight unit with a liquid crystal panel. In this case, to respond to frequency change of the input VSYNC signal, the backlight driver sets an output period by calculating an input period of the VSYNC signal on a per frame basis and generates internal clocks required to generate duty of the PWM signal using the output period of the VSYNC signal.
However, with regard to calculation of the input and output periods of the VSYNC signal on a per frame basis, if sudden frequency change of the VSYNC signal occurs, conventional backlight drivers may fail to set an output period depending on the suddenly changed input period, thereby having difficulty in generating the internal clocks. This causes the duty ratio of a PWM signal to deviate from a desired value. Consequently, the LED backlight unit exhibits brightness fluctuation, thereby suffering from deterioration of image quality, such as occurrence of flickering on a screen.